SDLP Mid-Ulster councillor Denise Mullen has quit the party and joined new all-Ireland party Aontú.
Ms Mullen decided to leave on the eve of a same-sex marriage vote at Mid-Ulster District Council on Thursday evening.
She disagreed with her party's decision to oppose a DUP motion on same-sex marriage.
DUP councillors had proposed a motion that the council oppose the decision taken at Westminster to introduce same sex marriage in Northern Ireland if Stormont is not restored by October 22.
The SDLP councillors were intending to vote against the motion.
DUP Councillor Paul McClean also called on the council to write to new Prime Minister Boris Johnson to call on him to overturn the decision.
"I have been considering it (resigning) for a while," Ms Mullen told the Irish News.
"The timing is right for me to continue to do my work for constituents and others and not under the heading of the SDLP.
"I wish them well in whatever direction they are trying to go. I feel abstaining was best while, as a Catholic, not having a consultation/direction from the Catholic Church, I don't honestly have a view either way."
In a later statement Ms Mullen confirmed she had joined Aontú, led by Peadar Tóbín TD.
"I am delighted that I have made the decision. Like so many people, I have become angered and disillusioned by the radical change in direction of the SDLP leadership in recent times," she said.
"The SDLP leadership no longer represent the party grassroots. Issues such as Irish Unity, Economic Justice and the right to life are being dropped or seriously changed.
"The leadership of the SDLP are now firmly pro-abortion. That's not good enough for me or many others."
A SDLP spokesperson said: "Ms Mullen stood and was elected on a clear SDLP ticket just twelve weeks ago. SDLP policy was clear then as it is now. The party has accepted Ms Mullen’s resignation and wishes her well."
Peadar Tóibín TD, leader of Aontú has said: “We are delighted that Denise has joined our new political movement. Denise has been a phenomenally active public representative for the people of Dungannon."
Ms Mullen was first elected to the council in 2014.
The SDLP website states that she comes from a "staunch SDLP family" and sat on the Party Executive.
Her father Dennis Mullen was murdered by the UVF in 1975.